This invention relates to a method and apparatus for estimating a propagation path. More particularly, the invention relates to a propagation path estimation method and apparatus for estimating a propagation path traversed by a transmit signal in a receiver when communication utilizing OFDM (Orthogonal Frequency Division Multiplexing) is performed.
Frequency-selective fading ascribable to a multipath environment occurs in wideband wireless communications. An effective method of dealing with this is multicarrier modulation, which divides the transmission bandwidth into narrow bands (subcarriers) that do not undergo frequency-selective fading, and transmits the subcarriers in parallel.
At present, specifications regarding digital TV and audio broadcasts (in Japan and Europe) and wireless LAN (IEEE 802.11a) are being standardized based upon OFDM transmission, which is one type of multicarrier modulation. An OFDM-based modulation scheme has been proposed for next-generation mobile communication systems as well.
With a wireless communications system that employs OFDM-based modulation, it is necessary to estimate the propagation path characteristics (propagation path information) of all subcarriers. The precision of the estimation has a major effect upon transmission error rate in a manner similar to that of other wireless communications systems that use coherent detection. For this reason, a wireless communications system using OFDM-based modulation transmits a known symbol on a subcarrier used in transmission and estimates propagation path information subcarrier by subcarrier. As mentioned above, the precision of propagation path estimation has a major effect upon the transmission error rate and hence there are many cases where use is made of a technique that suppresses background noise contained in a propagation path estimation value estimated using a known symbol, or a so called pilot symbol. For example, a first prior-art technique is to average frequency between adjacent subcarriers [see Hiroyuki Atarashi, Sadayuki Abeta and Mamoru Sawahashi, “Performance of Forward Link Broadband Packet TD-OFCDM with Iterative Channel Estimation”, Technical Report of IEICE., DSP2000-154, SAT2000-110, RSC2000-186 (2001-01)], and a second prior-art technique is forced zero substitution of an impulse-response group on an estimated propagation path (see JP2000-341242).
The first prior-art technique performs averaging between adjacent subcarriers utilizing coherence (uniformity) in the frequency direction, thereby suppressing background noise. For example, if we let h1 to h512 represent the propagation path characteristics of 512 subcarriers, as shown in FIG. 32, the propagation path characteristics of three adjacent subcarriers are averaged and the average is adopted as the propagation path characteristic of the middle subcarrier. The first prior-art technique utilizes a certain property, namely that if the propagation path characteristics in a coherent bandwidth that is proportional to the reciprocal of delay spread are coherent and M-number of subcarriers exist in this coherent bandwidth, then the propagation path characteristics of these M-number of subcarriers will be the same. The first prior-art technique is such that if the delay spread is small, the amount of fluctuation in the propagation path characteristics along the frequency direction is slight (correlation is large) and therefore background noise can be suppressed effectively by increasing the number of averaging operations in the frequency direction. With regard to the definition of delay spread, a difference develops between the arrival times of received waves in a multipath environment. The spread between these delay times is referred to as delay spread.
In the first prior-art technique, however, correlation between amounts of channel fluctuation between adjacent subcarriers diminishes as delay spread increases. Consequently, a problem which arises is that estimation precision declines if the number of averaging operations along the frequency direction is made greater than necessary. Actual delay spread involves a great deal of fluctuation and, in an outdoor environment, can be 0.2 to 2.0 μs in urban areas and 10 to 20 μs in mountainous areas. This means that with the first prior-art technique, it is necessary to select the optimum number of averaging operations while measuring delay spread. Further, even if the optimum number of averaging operations has been selected, a problem which arises is that averaging cannot be performed in an environment where the delay spread is large, as in mountainous areas, and background noise will not be suppressed and receiver performance will be degraded compared to the without-averaging technique.
The second prior-art technique compares the power of an impulse-response group on an estimated propagation path with a predetermined threshold value and forcibly substitutes zero for impulses that are below the threshold value, thereby suppressing background noise. An OFDM signal is such that a signal that has been mapped to a subcarrier is transmitted upon being converted to the time domain by IFFT processing. However, if the IFFT size (N-point IFFT) and number (Nc) of subcarriers used in signal transmission differ, this is equivalent to performing multiplication by a rectangular window on the frequency axis. As a result, a time signal in OFDM is a signal of a convoluted time response function decided by the number (Nc) of subcarriers used. If the subcarriers at the edges of the spectrum is not used for transmission, time response is followed by a sinc function. Under the condition that time response is the sinc function, the second prior-art technique utilizes this feature to set the threshold value to a value that is approximately 13 dB below the main lobe, thereby arranging it so that a side lobe of the sinc function will not be discriminated as a valid path (impulse).
If N-point IFFT processing is executed with N items of data serving as the components of N-number of subcarrier components f1 to fN, the frequency spectrum is as indicated at (A) in FIG. 33. In OFDM, a signal that has undergone IFFT processing is converted to an analog signal, baseband signal components of f1 to fN are extracted from the analog signal by a low-pass filter, and these are up-converted to radio frequency and transmitted. In order to select baseband signal components of f1 to fN, a low-pass filter having a sharp cut-off characteristic is necessary. Fabricating such a filter, however, is difficult. Accordingly, carriers on both sides of the N-number of subcarriers f1 to fN are not used in data transmission, i.e., Nc-number (Nc<N) of subcarriers are used in data transmission, as illustrated at (B) in FIG. 33. When the number Nc of subcarriers used in data transmission and the IFFT size (=N) thus differ, the propagation-path response becomes a sinc function and not an impulse and the peak value of the main lobe diminishes to Nc/N, as illustrated in FIG. 34. Consequently, in a case where Nc=N holds, the propagation-path response becomes an impulse, as illustrated at (A) in FIG. 35, but if Nc<N Holds, it becomes a waveform on which the sinc function has been superimposed, as indicated at (B) in FIG. 35. The second prior-art technique sets the threshold value to a value that is approximately 13 dB below the main lobe, thereby suppressing background noise in such a manner that a side lobe of the sinc function will not be discriminated as a valid path (impulse).
In the second prior-art technique, the side lobes of the sinc function are eliminated and only the main lobe is discriminated as a valid path. However, since the amplitude of the main lobe diminishes to Nc/N owing to the nature of the sinc function, a problem with the second prior-art technique is a residual estimation error. Further, in propagation environment in which path spacing is small, interference develops between the side lobes of the sinc function, the combined value in the overlapped sample exceeds the threshold value and a path is erroneously judged to be present where no path exists.